It is the heavyweight championship of the world, the match that makes for appointment viewing even on this first Sunday of the NFL season, Roger Federer in one corner, Novak Djokovic in the other in the men’s final of the U.S. Open.

And even as these all-time athletes who own a combined 26 major championships (a record 17 for Federer) slug it out, so much of the anticipation is focused on whether the 34-year-old Swiss Maestro will dare use the second-serve sneak attack against Djokovic that he has employed to his advantage since unveiling it last month against the Serb in the final at Cincinnati.

It’s known as SABR (Sneak Attack by Roger), the tactic by which Federer rushes the net for a half-volley return. It’s serve-and-volley on its head. Truth be told, there doesn’t appear to be any great advantage to the scheme — er, strategy — except for this: It seems to annoy the heck out of some of his opponents, including the fellow he plays on Sunday.

Apparently some interested parties aren’t so sure the gentlemen’s gentleman is playing such a gentlemanly game. Aww.

Djokovic’s coach, Boris Becker, has gone on television and said the tactic is “Almost disrespectful.” Djokovic himself, an amiable and likeable fellow under essentially all circumstances, become short and brusque when asked about it.

“He tried that in Cincinnati. It worked a couple of times,” Djokovic said after eliminating ailing defending champion Maren Cilic in straight sets in their semifinal match. “It’s an exciting shot for him. For the opponent across the net, not so much.

“So I have nothing else to say about that.”

John McEnroe has had something to say about that. Shocker. From his perch in the ESPN booth, McEnroe has said he might have been insulted had an opponent tried that against him. Who knew Johnny Mac was such a sensitive soul?

Federer uses the approach judiciously. Actually, he uses it sparingly. But when he does, it makes an impression. It’s gotten a rise each and every time from the crowds at this Open through which the Maestro has been at the height of his powers, running through six opponents without dropping a set while holding serve 80 out of 82 times.

Novak Djokovic and his coach, Boris Becker, are not fans of Roger Federer’s SABR tactic.Reuters

Every time Federer faced a second serve in Friday’s 6-4, 6-3, 6-1 semifinal deconstruction of his friend and fellow Swiss Stan Wawrinka, the fans at Ashe moved forward in their seats, anticipating the People’s Choice charge to the net. There were excited utterances and noise, the kind of which McEnroe would have reveled in back in the day.

Federer used the tactic only a handful of times to mixed results. But it has now become every bit as much a part of his signature as his elegance, mobility and devastating forehand. Somehow, at 34, Federer is fitter, more mobile, more diversified in his game. Somehow, after all these years, Federer has come up with this wrinkle.

And as the People’s Choice — about as many stylized “RF” caps on the grounds as Mets caps in the ballpark across Roosevelt Avenue — attempts to extend his record by adding an 18th major to his resume, he is certainly not about to apologize for it.

“No, it’s not disrespectful,” Federer said on Friday when asked about Becker’s take. “Pretty simple.”

Federer hasn’t won a major since taking Wimbledon in 2012, while Djokovic has won four, including finals victories over his rival at Wimbledon both this year and last year. Moreover, this is Federer’s first trip to the Open final since 2009, when his run of five straight titles was halted by Juan Martin del Potro.

In the interim there were consecutive five-set semifinal defeats to Djokovic in 2010 and 2011, with Federer holding double match points in each.

They do it again on Sunday for the 42nd time, Federer holding a 21-20 head-to-head edge but down 7-6 in majors. They do it again in this classic matchup with world No. 1 Djokovic aiming for his third major championship of the year following victories at the Australian and Wimbledon, and a defeat in the finals to Wawrinka at Roland Garros.

Of course, he’s 1-0 against Djokovic since adding SABR to his arsenal.